Fuel cells are electrochemical devices in which a fuel and an oxidizer react to directly generate an electrical current. Fuel cells are silent and clean in operation and can provide power sources which have a high power to weight ratio. As a consequence, fuel cells are attractive energy sources for a large number of applications.
One class of fuel cells utilizes hydrogen as a fuel. The chemistry of such systems is relatively simple; however, their operation requires the storage and delivery of a gaseous fuel which can complicate the system. Another class of fuel cells utilizes organic liquids as a fuel. These liquids typically comprise methanol or other alcohols. Fuel storage and delivery in such systems is relatively simple. In some instances, liquid fuel cells utilize air as an oxidizer, and may be configured so that they are “air breathing” thereby eliminating the need for pumps or other gas delivery systems. Such liquid fuel, air breathing fuel cells can provide compact, mechanically simple power sources. However, presently implemented fuel cell bi-cell configurations have not been able to fully achieve all of the potential benefits of such systems.
One approach in the prior art to the fabrication of fuel bi-cell designs utilizes the “bipolar plate” design wherein a single bipolar plate serves as a current collector for both anode and cathode electrodes in two adjacent single cells. One surface of the plate is in contact with an anode of the cell and the other with the cathode. When electricity passes through the bipolar plate, electrical polarization occurs between the two sides thereof These plates are typically made of graphite, but in some instances they are fabricated from a metal sheet. The bipolar plate design provides a compact volume and high internal conductivity, together with a rigid, bi-celled structure; but, it has the disadvantages of requiring precise thermal and liquid flow management, which generally requires the use of fuel and air pumps. Consequently, such designs are expensive and difficult to operate. Some examples of prior art showing bipolar plate designs of fuel bi-cells are found in U.S. Pat. Nos. 5,776,624; 5,496,655; 5,798,188; and 6,284,401.
In other instances, the prior art has utilized fuel cell bi-cells with non-bipolar plates. In systems of this type, each current collector will serve only as an anode or cathode electrode in the fuel cell; and as a consequence, each cell in the bi-cell operates independently. The disadvantages of the bipolar design are high internal resistance, fragile bi-cell structure, low power output and fuel leakage. These non-bipolar designs are primarily used for hydrogen/air fuel cells and only occasionally in liquid fuel cell systems. Some prior art examples of non-bipolar designs are found in U.S. Pat. Nos. 5,709,961; 5,958,616; 6,132,895; 6,268,077; 6,194,095; and 5,958,616. In most instances, such non-bipolar designs are configured so that the single cells are arranged in a plane, and this type of a design is generally detrimental to achieving high power density outputs.
In most instances, high density fuel cell bi-cells require the use of pumps for delivering air or other oxidant thereto. The prior art has implemented several designs in an attempt to make fuel cell bi-cells directly air breathing so as to minimize cost and weight. However, prior art air breathing bi-cell assemblies have been found to be fragile and prone to fuel leaking and/or have poor electrical contact between the electrodes and current collectors. Some prior art approaches to the fabrication of direct air breathing fuel cells are found in U.S. Pat. Nos. 6,268,077; 5,645,950; 5,514,486; 5,595,834; 5,935,725; 6,040,075; and 5,709,961.
As will be described hereinbelow, the present invention provides a fuel cell bi-cell assembly which is simple in construction, rugged, and efficient. The bi-cell assembly of the present invention provides a very high power density, and may be configured to operate with a liquid fuel such as an alcohol, and to be directly air breathing. Furthermore, the system of the present invention is modular and allows for ready configuration of a series of fuel cells into mixed series parallel arrays so as to allow for the optimum control of the current and voltage output of the bi-cell, and long operational life. These and other advantages of the invention will be apparent from the drawings, discussion and description which follow.